Conventionally, as a battery charging method, there are a constant-voltage charging method which applies a constant voltage to a battery for charging, and a constant-current charging method which applies a constant current continuously to the battery for charging. Though there are both merits and demerits in either of these methods, it is not easy to realize shortening of charging time considerably by any of existing methods.
In recent years, from a view point of environment protection, practical use of a so-called electromobile vehicle which uses the batteries as a driving source is strongly desired. For wide use of the electromobile, while it is necessary to shorten the battery charging time a great deal, the battery charging method which complies with this request sufficiently is still not developed at present.
Incidentally, a capacity of a battery is defined by an ampere-hour capacity (Ah) and an hour rate. For example, when the battery capacity of 10 Ah/10 hour rate is discharged at a current of 1 A, it can be used continuously for 10 hours. When this battery is discharged at a current of 4 A, while the continuous usable time is 2.5 hours by calculation, it takes, as a matter of fact, approximately 1.5 hours to reach the charge-end voltage. In the same way, when discharged at a current of 10 A, while the continuous usable time is 1 hour by calculation, a practical continuous usable time is approximately 35 minutes.
Similarly, this applies to charging, too. For example, when a battery of 10 Ah/10 hour rate capacity is charged at a constant current of 1 A for 10 hours, it reaches fully charged state. When it is charged at a constant current of 10 A, the chemical reaction in the battery proceeds faster than the calculation, and as a matter of fact, it approaches to the fully charged state by a half of the calculated time of one hour.
However, when the charging current is simply increased, it is liable to be overcharged and there is possibility of damaging electrodes of the battery. Moreover, there is a problem that only the surface of the electrodes is charged and the charging is not penetrated into the electrodes.
The present invention has been devised in view of the above-mentioned problems. It is the object, therefore, to provide a battery charging method and its apparatus which is able to charge in a very short item and to penetrate the charging into the electrodes without damaging a battery.